This invention is related to piezoelectric materials and, more particularly, to a 0-3 PZT-polymer composite for transducer applications and the like.
Piezoelectricity can be explained as electric polarization produced by mechanical stress in certain substances. Piezoelectric materials, such as lead zirconate titanate (PZT), are used in a wide variety of applications. In hydrophone devices piezoelectric materials detect low frequency acoustic waves passively. Hydrophones are frequently made from single phase PZT. Large hydrostatic piezoelectric charge and voltage coefficients (d.sub.h and g.sub.h) are desired in these devices. Thus, even though the magnitudes of the piezoelectric coefficients d.sub.33 and d.sub.31 of PZT are large, the hydrostatic coefficients d.sub.h and g.sub.h are small, because the d.sub.33 and 2d.sub.31 coefficients are almost equal and opposite in sign, and also, the dielectric constant of PZT is large. The large difference of the acoustic impedance between PZT and water requires impedance matching layers for underwater hydrophone applications.
In order to improve and modify material properties for hydrophone devices, several different types of piezoelectric PZT-polymer composites have been recently investigated utilizing the concept of phase connectivity. It has been found that the electric flux pattern and the mechanical stress distribution together with the resulting physical and piezoelectric properties depend strongly on the manner in which the individual piezoelectric and polymer phases of the diphasic composites are interconnected. Each phase in a composite may be self-connected in zero, one, two, or three dimensions. Thus, a diphasic 2-1 connectivity pattern, for example, has one phase self-connected in two dimensional layers, the other in one dimensional chains or fibers. Below are represented some of the composites with different connectivity patterns in which the piezoelectric phase appears first.
0-3 composites: PZT particles suspended in a polymer matrix PA0 1-3 composites: PZT rods aligned in the poling direction held together by a polymer matrix PA0 1-3-0 composites: PZT rods aligned in the poling direction held together by a foamed polymer matrix PA0 3-1 and 3-2 composites: holes drilled in a prepoled PZT block, then the holes filled or covered by polymer. PA0 3-3 composites: lost-wax method using coral as the starting material, or by a fugitive phase method (BURPS process).
The d.sub.h, g.sub.h coefficients and d.sub.h g.sub.h figure of merit of the diphasic composites are significantly improved over single phase PZT due to decoupling of the d.sub.33 and d.sub.31 coefficients and/or the reduction of the dielectric constant.
The piezoelectric ceramic-polymer composites of 1-3,1-3-0, 3-1, 3-2, and 3-3 connectivities are often expensive and cumbersome to fabricate. The PZT-polymer 0-3 composite is relatively easy and inexpensive to make. However, in 0-3 composites, early studies showed that the PZT particles should have a diameter greater than the thickness of the composites to obtain sufficient poling. For smaller particles of PZT, very large poling field strengths (.perspectiveto.100-150 kV/cm) are needed to achieve sufficient poling.
For a 0-3 composite consisting of spherical grains embedded in a matrix, the electric field E.sub.1, acting on an isolated spherical grain is give by ##EQU1## In this equation, K.sub.1 and K.sub.2 are the dielectric constants of the spherical piezoelectric grains and the polymer matrix, respectively, and E.sub.0 is an externally applied electric field. For a 0-3 composite of PZT powder and polymer, K.sub.1 is about 2000 and K.sub.2 about 5. In such a composite with an external field of 100 kV/cm, the electric field acting on the piezoelectric particles is only about 1 kV/cm which is insufficient to pole the composite. According to the above equation E.sub.1 .about.E.sub.0 only when the dielectric constant of the piezoelectric phase approaches that of the polymer phase. Most of the ferroelectric materials have very high dielectric constants and hence the above condition cannot be satisfied.
One way to meet this poling difficulty is to raise the polymer matrix conductivity. In Japanese Kokai No. 56-93383, July 28, 1981, a piezoelectric material is prepared by dispersing fine piezoelectric ceramic powder in a high permittivity polymer matrix consisting of an insulating polymer and an organic substance with high conductivity.